1. Field of the Invention
This invention relates to electrically conductive coatings and more particularly relates to electrically conductive coatings on rigid transparent substrates such as glass or flexible plastic substrates. In a more particular aspect, the present invention relates to heated windows, particularly heated motor vehicular windows employing such substrates.
2. Brief Description of the Prior Art
U.S. Pat. No. 3,698,946 to Kaspaul et al discloses transparent, electrically conductive coatings on glass or flexible plastic substrates. The coatings are a composite layer comprising a first layer deposited as titanium monoxide, an intermediate layer of metal which may be copper, silver, gold, palladium or tin, and a third layer deposited as titanium monoxide. The coated articles are disclosed as being useful in solid state photo detectors, light emitting devices, image converters and image amplifiers. Unfortunately, these coatings give the coated article a combination of relatively high sheet resistances and low luminous transmittances. Typical sheet resistances and luminous transmittances set forth in the Kaspaul et al patent for coated glass and plastic substrates are about 1,600 to 200,000 ohms per square and 38 to 76 percent, respectively. These properties make the coatings undesirable for motor vehicle windshields which require a combination of high luminous transmittances, on the order of 80 percent or more, and low sheet resistances, on the order of about 1 to 10 ohms per square, to develop useful amounts of heat obtainable with motor vehicle generator voltage.
It has been found that the luminous transmittances of the coated articles described in the above paragraph can be improved if the titanium oxide layers are deposited as titanium dioxide. Such films are colorless, whereas films deposited as titanium monoxide transmit blue. However, films deposited as titanium dioxide are surprisingly not compatible with the subsequently deposited silver layer. The silver film, although initially continuous and of high conductivity, breaks down rapidly and becomes non-continuous, resulting in a drastic increase in resistance and lowering of the luminous transmittance over a short period of time, i.e., less than 24 hours. Surprisingly, it has been found that when the layers of titanium oxide are deposited as TiO.sub.x, where x has a value within the range of 1.3 to 1.7, the subsequently applied silver film remains stable and highly conductive, and the TiO.sub.x film has the facility to become colorless upon exposure to normal atmosphere of air. When the film is deposited as titanium monoxide, the film does not readily clear, and remains blue.